My girl Rylie is 8 months old next week and she went through the exact same thing, my boyfriend made fun of her and use to call her an old lady when she was still only4 months old, cuz her hair on her head was turning grey.
He still calls her an old lady and her hair on her head is still light but the rest of her body has pretty much remained the same, she is getting little streaks for silver down the spine of her back and other few spots but is mainly very dark.
I've added some pics so you can see her hair change, there a little out of order
1. When she was about 3 1/2 months old.
2. Was taken this past August.
3. Was taken in June
4. Was taken in July... you can really see how light her head is in this one.
I still have no idea what colour she will end up once she's older.